1. Field of the Invention
This invention pertains an ultraviolet fire detection device for detecting fires which uses a phosphor to translate energy from ultraviolet wavelengths down into visible and near infrared wavelengths for detection by a photo-sensitive solid state device which can sound an alarm upon detection of the ultraviolet wavelengths associated with flames. In one embodiment a two window approach is used, the reference window being opaque to ultraviolet wavelengths. In the second embodiment, a prism or diffraction grating is used to separate the light source into its various wavelengths.
2. Description of the Prior Art
In the present state of the art, electro-optical sensing devices, such as silicon solar cells (photovoltaic cells), cadmium selenide and the like are not responsive to light in the ultraviolet region of the spectrum. Those photo-sensitive cells which are enhanced to be sensitive to ultraviolet wave-lengths are also extremely sensitive to emissions in the visible and infrared ends of the spectrum and thus lack the capability of discriminating ultraviolet emissions. Hence, there are no solid state electro-optical devices which can serve as ultraviolet detectors in fire detection devices.
In the present state of the art in ultraviolet fire detection devices, photodiodes are used for detection of ultraviolet light. The vacuum photodiode is essentially a vacuum tube with cesium sulfide or some other cesium salt in it, and it is operated at a very high voltage (from 200 to 300 volts). When a photon enters the quartz envelope of the photodiode, it causes an ionic avalanche of current between the two electrodes of the photodiode. This current is then passed through a resistor which creates a voltage drop. The voltage drop is then amplified and converted into a detection signal.
Photodiodes present many problems in fire detection systems. As vacuum tubes, they are somewhat unreliable. An ultraviolet fire detector must be able to operate under a wide range of temperature extremes. Quartz-enveloped photodiodes are not rugged and cannot withstand temperature extremes. They are susceptible to static electricity, even small amounts. They are too sensitive and will respond to lightening, arc welding and similar phenomena, thus causing false alarms. Because of these problems, a fire detection system using photodiodes must include an elaborate test and diagnostic procedure to be performed regularly to discover devices whose photodiodes have failed. The reliability of a tube degrades raidly because of the loss of hermiticity. When the tube fails, there is no indication of failure from the detector. As the tube loses its vacuum over time, it becomes less and less sensitive. Even though it works with a test lamp, it can be so insensitive as to be useless.
To solve the problems of present ultraviolet fire detection systems, the present invention provides a solid state detection system which eliminates the inherent unreliability of the photodiode vacuum tube and provides a means to translate ultraviolet wavelengths of light to visible wavelengths which can be easily detected by photo-sensitive solid state devices. One can make the photodiode sensitive to any desired frequency from 200 nanometers to 1500 nanometers by adjusting the voltage across the diode and adjusting jthe phosphor element inside the photodiode. The frequency is inversely proportional to the wavelength; the higher the frequency, the higher the energy and the shorter the wavelength. Silicon photovoltaic cells are available which are responsive from 300 to 1000 nanometers. Sunlight is filtered out at 280 nanometers by the ozone layer which acts as a low-pass filter. This layer filters out ultraviolet radiation above 280 nanometers. Thus, if a photosensitive solid state device such as a silicon photovoltaic cell is responsive to 300 nanometers and lower in frequency or from 300 nanometers to 1500 nanometers in wavelength, then the cell is responsive to sunlight. If the cell were used in a fire detection system, the sunlight would be a source of false alarms. To overcome this problem, the present invention utilizes a hosphor layer in conjunction with a solid state photosensitive device, the phosphor serving to translate the wavelength of the ultraviolet radiation into a longer wavelength which can be reliably detected by a photosensitive solid state device.
Various phosphors are available which will fluoresce when exposed to ultraviolet radiation, which has wavelengths shorter than 280 nanometers. In particular one phosphor fluoresces from 210 to 290 nanometers, which is an acceptable range for ultraviolet fire detectors. The phosphor receives an ultraviolet photon which has a relatively high energy and captures it in its crystal structure. The phosphor removes some of the energy and reemits the photon. The reemitted photon, having less energy comes out at a different wavelength of light. The energy surplus is dissipated in the crystal as heat. The phosphor receives irradiation in the ultraviolet range and will glow with a yellow-green color, even though the ultraviolet radiation is imperceptible. The wavelength of light emitted from the fluoresced phosphor is sufficiently long enough to be detected by a solid state photosensitive or photodetection device. Thus, the phosphor translates ultraviolet light in the 220 nanometer range to a yellow light which is in the 580 nanometer range. This latter range is in the middle of the response curve for some silicon photovoltaic cells which are very responsive to light in this color range. Thus, the photovoltaic cell can be made responsive through this translation of the ultraviolet light in the 220 nanometer range to a yellow light in the 580 nanometer range, even though the photosensitive device is not responsive to the untranslated ultraviolet light. Thus, using a phosphor as a wavelength translation means, one can characterize the general band of wavelengths desired, for example, the ultraviolet energy emitted from a flame.
The present invention provides two embodiments which use the ultraviolet wavelength translation phenomenen discussed above in solid state ultraviolet fire detectors. In the first embodiment, two windows are utilized, one of glass and one of quartz. Glass is used in the reference window because it is opaque to ultraviolet radiation. A layer of phosphor is deposited behind the both windows. A photosensitive device is positioned behind each window. The outputs of the photosensitive devices are compared in an operational amplifier. In the presence of normal ambient light, sunlight or artificial illumination, both photo-sensitive cells will detect the same amount of light energy, both outputs are the same, and no alarm will be sounded. However, if there is a flame in the environment, the ultraviolet radiation from the flame will pass through the quartz window, cause the phosphor to fluoresce and cause the resulting yellow light to be detected by the photosensitive cell behind the quartz window. At the same time, the glass window which is opaque to ultraviolet radiation will not permit the phosphor behind it to fluoresce, but will detect only the ambient light in the environment. Hence, the glass window and its phosphor layer will cause its photosensitive cell to send a reference signal relative to the ambient light to the operational amplifier. Since the photosensitive cell behind the quartz window will detect both the ambient light and the yellow light from the fluorescent phosphor, this photo-sensitive cell will send a different signal with a greater output to the operational amplifier. The operational amplifier will then cause an alarm.
In the second embodiment, phosphor is also used as the ultraviolet wavelength translation medium. In this embodiment, a quartz prism or diffraction medium is used to separate the wavelengths of light and to project them on a phosphor coated screen. A photosensitive cell positioned behind the screen will detect the fluorescense of the phosphor in the presence of ultraviolet radiation and its output will then exceed a threshold level, causing an alarm to sound. Using a plurality of cells behind a plurality of locations on the prism or diffraction medium, the photosensitive detectors can detect various selected wavelengths of light and thus distinguish between different types of fires such as a hydrogen fire, a butane fire or a propane fire. The fire detection device can be further utilized to select the appropriate extinguishing agent for the type of fire detected.
In all embodiments, the invention used solid state photosensitive devices to detect ultraviolet light of specific frequencies by using a phosphor to translate the wavelength of the ultraviolet radiation to a wavelength in the response range of the photosensitve device. Changes in basic ambient light are ignored by use of a reference cell. When ultraviolet light is present in the spectrum, that light impinging on the quartz sampling cell will be enhanced by the emissions from the phosphor, this enhancement providing a greater output causing the operational amplifier to sound an alarm.